The present invention relates to an optical axis adjusting apparatus for an electron microscope, and more particularly to such an apparatus for aligning the optical axis of a radiation lens system and the optical axis of an imaging lens system with each other.
Generally, when the optical axis of the radiation lens system is titled with respect to that of the imaging lens system, the adjustment of the optical axes including the optical axis alignment through current change (current alignment) or the optical axis alignment through voltage change (voltage alignment) is carried out. Namely, if an objective lens current or an accelerating voltage is changed, a final image moves circumferentially or radially about a certain point, called a current center or voltage center, which does not move. The optical axis adjustment is performed so as to position the current or voltage center to a center of the final image or usually the center of a flourescent screen to which the final image is projected for observation.
In the above-mentioned adjustment method, however, since the image moves in a circumferential direction or radially about the current or voltage center in accordance with the change of the objective lens current or accelerating voltage, it is hard to recognize the current or voltage center among the movement of the image being observed. Further, the image movement in accordance with the current or voltage change accompanies the blurring of the image. This makes it more difficult to find out the current or voltage center. Thus, the optical axis adjustment requires a great deal of skill.
On the other hand, if the period of the current or voltage change is quickened to utilize an after-image of the flourescent screen being observed, the movement of the image is not be observed but only the blurring of the image is observed. In that case, therefore, it may be possible that the current or voltage center can be easily found out. But, the period of the current or voltage change is limited to several hertz at the highest because of a large time constant of the objective lens coil or the accelerating voltage circuit. Actually, therefore, any movement of the image is involved or observed and hence the current or voltage center cannot be easily found out.